import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringBuilder output;
		String input;
		String[] spInput;
		String result;
		int upper, lower, height;
		double area;
		
		while((input = br.readLine()) != null) {
			spInput = input.split(" ");
			upper = Integer.parseInt(spInput[0]);
			lower = Integer.parseInt(spInput[1]);
			height = Integer.parseInt(spInput[2]);
			
			area = ((upper + lower) * height) / 2;
			
			result = Double.toString(area);
			result = result.substring(0, result.indexOf(".") + 2);
			output = new StringBuilder("Trapezoid area:" + result);
			
			System.out.println(output);
		}
	}
}